Some MEMS devices can sense both linear acceleration and angular rate. Such sensors, which are a type of “multisensor,” typically sense displacement of a mass using sensing capacitors.
One such MEMS multisensor is described in U.S. Pat. No. 6,845,665 entitled “Micro-Machined Multi-Sensor Providing 2-Axes of Acceleration Sensing and 1-Axis of Angular Rate Sensing.” The multisensor in that patent includes a rigid frame anchored to a substrate, and a pair of a proof masses (or “shuttles”) suspended from the rigid frame by a plurality of flexures. The shuttles may be vibrated along one axis, and Coriolis acceleration may cause the shuttles to be displaced along another axis. Vibration of the shuttles is typically forced by drive capacitors that are driven by alternating voltages to create electrostatic drive forces, while the displacement of the frame is sensed by separate sensing capacitors. The device detects linear acceleration by sensing the displacement of the frame in two axes, and senses Coriolis acceleration by sensing the rotation of the frame about a perpendicular axis. Therefore, the multisensor detects both linear acceleration and (because the Coriolis acceleration is a function of the rotation of the multisensor) rotation.
Some sensors, such as some gyroscopes, include two accelerometers vibrating in antiphase, so that the responses to Coriolis acceleration are in antiphase, and the responses to linear acceleration are in-phase. In such a sensor, Coriolis acceleration sensing can be improved by mechanically coupling the two accelerometers so that the structure provides little resistance to antiphase vibration, but substantial resistance to in-phase motion. U.S. Pat. No. 7,347,094, entitled “Coupling Apparatus for Inertial Sensors” describes fork members configured to couple two masses.
However, because a linear acceleration along at least one axis would cause both accelerometers to displace in the same direction (that is, in-phase), a sensor with a dampened in-phase response would have diminished linear acceleration sensing capacity.